The present invention relates to the field of power electronics. It relates in particular to a converter system as claimed in the precharacterizing clause of claim 1, and to a method for operating such a system.
When a number of voltage intermediate circuit converters with the same intermediate circuit voltage level are used in an industrial system, it may be advantageous to form a system with coupled voltage intermediate circuits, referred to as a common DC-bus system, rather than using individual converters, each based on a power supply system converter, a voltage intermediate circuit and a load converter. This common DC-bus system comprises power supply system converter modules (passively with diodes or actively with controlled semiconductor switches), one or more intermediate circuit modules and load converter modules (for example motor feed, generator feed, etc.).
The advantages of this system are, for example, that power can be interchanged freely between the load modules without any necessity to use bidirectional power supply system converter modules, the use of common auxiliary devices (charging apparatus for the intermediate circuit, overvoltage limiters, brake controllers, etc.), and the capability to use relatively large, low-cost modules as power supply system converters. If, for example, there are five load modules which each feed their load with a power of 100 kW, a single power supply system converter module of 500 kW can supply the system. This saves not only converter costs, but possibly also transformer costs.
However, a system such as this also has disadvantages, which need to be counteracted by appropriate measures. One important problem in this case is protection of the system when one of the converter modules fails. If all the modules were to be coupled to the common intermediate circuit without any further measures, a short circuit in a single module would lead to the common intermediate circuit being short-circuited, and thus to the failure of the entire system. Furthermore, the amount of energy in the intermediate circuit, which is higher than that in an individual converter, can lead to damage in the defective module, which assumes a far greater extent than would be the case in an individual converter.
One normal solution to this problem is to couple the individual converter modules to the common intermediate circuit via a protective device. This limits the destructive amount of energy which is introduced into the defective module and isolates the module from the rest of the system, so that it is possible to continue to operate the rest of the system. However, this solution has significant disadvantages: firstly the protective devices act so slowly that this interferes with the operation of the other modules.
In general, it is impossible to avoid a brief interruption in operation in this case. Secondly, the protective device is destroyed and must be replaced. This increases the repair time, results in additional repair costs, and also increases the spares stockholding costs.
The object of the invention is thus to provide a converter system in which a defective converter module is selectively isolated from the common intermediate circuit, in which interruption-free further operation of the other modules on the intermediate circuit is possible, and in which the isolating means are not destroyed, and to specify a method for its operation.
The object is achieved by the totality of the features of claims 1 and 9.
The essence of the invention can be described by two steps:
First Step:
Each converter module has a reasonable proportion of the required DC voltage intermediate circuit capacitance allocated to it (xe2x80x9cdistributed intermediate circuitxe2x80x9d). In this case, the subdivision is typically based on two criteria: the capacitors (xe2x80x9clocal intermediate circuitxe2x80x9d) allocated to one converter module should be at least sufficiently large that they can carry the alternating current introduced into the intermediate circuit by this module; and the energy which is stored in the capacitors allocated to one converter module must not exceed the level at which unacceptable destruction would be caused if these capacitors were to discharge into the faulty module.
Apart from the capacitors in the local intermediate circuits, there are no further DC voltage capacitors in the intermediate circuit having a capacitance which is in the same order of magnitude as the capacitances of the locally installed capacitors.
Second Step:
The converter modules with a local intermediate circuit are coupled via at least one protective switch (semiconductor switch) to the DC voltage busbars. In this case, the protective switch is arranged such that it can interrupt any current flow from the positive DC voltage busbar to the positive connection of the local intermediate circuit, and/or any current flow from the negative connection of the local intermediate circuit to the negative DC voltage busbar.
According to a first preferred refinement of the invention, at least one of the converter modules is a converter module which exclusively transfers power from its AC voltage terminals to the DC voltage intermediate circuit, and the protective switch for this converter module is a semiconductor diode. This allows circuitry control complexity to be correspondingly reduced.
According to a second preferred refinement of the invention, at least one of the converter modules is a converter module which transfers power from the DC voltage intermediate circuit to its AC voltage terminals, and the protective switch in this converter module is a controllable power semiconductor switch.
If at least one of the converter modules is a multipoint converter module, the associated local intermediate circuit has a series circuit of capacitors whose junction points form corresponding intermediate points. In principle, multipoint converter modules as well need be coupled to the system only via the positive and the negative busbar, since the intermediate points are formed locally. This reduces the complexity for the DC voltage busbars considerably in comparison to a system having a centrally arranged capacitor unit. Furthermore, it is thus possible to operate converter modules with different numbers of points (for example a three-point converter and a four-point converter) at the same time in the system.
If a number of converter modules are in the form of multipoint converter modules, it is, however, also possible in order to make it easier to stabilize the intermediate points to provide appropriate intermediate point busbars in order to connect the intermediate points in the local intermediate circuits. In particular, if the number of busbars (DC voltage busbars and intermediate point busbars) is n, protective switches should be arranged in at least (nxe2x88x921) junctions between the local intermediate circuits and the busbars. For example, in a system having a number of three-point converters, this would result in three busbars, so that it would be necessary to use two protective switches for each module.
Fast interruption of the connection between the DC voltage busbars and the defective module makes it possible to continue to operate the rest of the system in the event of a fault. If the converter modules are connected, together with the local intermediate circuits and the protective switches, via additional isolating means to the busbars, which isolating means allow DC isolation between the converter modules and the busbars, the defective module can be DC-isolated from the system when a defect occurs, can then be repaired, and can then be reconnected to the system.
Further embodiments are described in the dependent claims.